The present invention relates to atomizing nozzles and, more particularly, to twin-fluid atomizers comprising features of double-dipped fuel/gas mixing and pintle self-cleaning for creating sprays with extremely fine drops.
Liquid atomization is one of the most effective methods in preparing liquid with maximized total surface area for various industrial applications, such as agricultural spraying, evaporation cooling, slurry drying, scrubbing of stack gases, dust collectors and oil-burner combustion processes. There are two kinds of atomizing schemes being used in nozzle design: pressure atomizers (single-fluid) and twin-fluid atomizers. The pressure atomizer, single-fluid, achieves droplet atomization by transforming pressure energy of the liquid to form high velocity liquid jet/film as it is injecting out of the atomizer. The exiting high velocity-jet/film is further sheared into small drops by the ambient airfield that contains induced-turbulent energy adjacent to the atomizer exit. This atomizer is widely used in low flow rate applications. In high flow rate requirements, however, the high velocity jet/film from a pressure atomizer becomes much thicker, which makes it harder to be atomized by the ambient air only. A remedy is to use a twin-fluid nozzle, which introduces pressurized gas to mix with the liquid prior to its injection, thus improving atomization at higher flow rate conditions. While in its operation, in the microscopic view point, gas is introduced under pressure to stir and mix with the liquid in the nozzle chamber to generate numerous tiny bubbles of gas entrapped into the liquid, which causes the viscosity and surface tension of the liquid to be much reduced (bubble-laden fluid) and results in much finer sprays. Technically, there are two atomization mechanisms involved in this liquid break-up process. The primary atomization is achieved at the nozzle exiting port by the sudden expansion of those entrapped-bubbles in the liquid as they experience pressure reduction, thus forming a fast moving dense spray of fine drops. The secondary atomization is subsequently introduced by the turbulent shear force from ambient air that breaks the high velocity moving drops into even finer sprays. The latter process shares the same spirit of the atomization mechanism with the pressure atomizer as described above. In general, the twin-fluid nozzle has broader usage in industrial applications in light of its much higher flow rate capacity and its much finer drops generated over a fairly wide operating range (also called turndown ratio).
On the twin-fluid nozzle, a fairly effective design of the prior art is shown in FIG. 8. This nozzle utilizes a nozzle cap 1000 to assist in the production of liquid drops. In FIG. 8, the nozzle cap 1000 includes an outer frame 1005, a pintle 1010 and support spokes 1015 to support and couple the pintle 1010 to the outer frame 1005. The pintle comprises an inlet splash plate 1020, a tapered shaft 1025 and an outlet deflector plate 1040 (FIG. 9). The function of the prior art is to make a liquid stream injecting on the splash plate 1020 perpendicularly to form liquid films on both surfaces of plate 1020 and spokes 1015. The swirling atomization air introduced from the upstream of the splash plate (not shown) is then mixed with the liquid films in the passage between the spokes as well as the downstream annular passage defined by the outer surface of the pintle 1025 and the inner surface 1035 of the frame 1005. Another example of prior art is shown in FIG. 10. This design modifies the prior art of FIG. 8 by positioning dams 1051 on spokes 1052 to improve the nozzle performance by reducing the amount of liquid flowing on the spokes while the liquid and air is mixing in the nozzle.
These designs are fairly effective in achieving gas/liquid mixing and atomization, nonetheless, subject to several limitations.
1. When the nozzle is used for injecting liquid with abrasive particles or contamination, erosion on the spokes 1015 or 1052 can occur, resulting in the damage of the pintle leading to failure of the nozzle.
2. As the swirling air being introduced into the mixing chamber of the nozzle (not shown) mixes with the liquid on the surface of both the splash plate 1020 and the spokes 1015, several aerodynamic wakes could be generated at the downstream of these spokes. In the wake region of the nozzle chamber (downstream of the spokes 1015), both velocity and angular momentum of the mixed fluid are significantly reduced in quantity and their distributions could become non-axial-symmetrically skewed. The skewed flow pattern then propagates through the nozzle exit and results in non-uniform sprays. This outcome can severely compromise the nozzle performance in several widely used applications, for instance, in furnaces of industrial oil burners, given the fact that the uniformity of a spray as well as its well maintained angular momentum are vital factors to stable flames in the burner.
3. As a spray is formulated after impinging on the deflect plate 1040, an axially symmetric recirculation region with lower pressure will also be formed in the center of the spray adjacent to the surface 1040 of deflector 1039. In this low-pressure recirculation zone, fine drops in the spray will be sucked back toward the downstream surface of the deflector and form large drops on the surface, called re-attachment. This process will compromise the spray quality quite severely in some cases. For example, in the applications of oil burner combustion or slurry heating processes, as the radiation heat in the furnace raises the surface temperature of the deflector, some recirculating fine drops in the spray accumulated on the downstream surface 1040 of the deflector can form layers of dried shells/cokings. Over time, the hardened slurry build-up, or coking layer in the oil burner cases, on top of the deflector edge can round and dull the sharp edge and cause the spray angle to be reduced, leading to more coarse drops in the spray. Nozzles under this limitation can compromise the quality of the powder-production in the slurry drying processes. Or it could severely damage the liner of a burner and cause unstable flames. The built-up coking layer on the pintle surface in the oil-burner will further cause hot spots on the pintle surface itself and eventually damage the pintle and cause the nozzle to fail.
This design comprises a vortex-mixing-module containing two new features. First, liquid and gas streams are pre-mixed by injecting both into the same swirler slots prior to their entering the annular mixing chamber of the module. Second, a pintle is center-mounted, and is provided with a self-cleaning feature. With this double-dipped mixing arrangement, the effectiveness of mixing between liquid/gas is much enhanced and the size of the mixing module can be greatly reduced, in comparison to the prior art, to result in more uniform fine sprays of great turndown ratio. The center-mounted pintle concept totally eliminates the possibility of pintle damage caused by the spoke erosion as shown in the prior arts (FIGS. 8 and 10) and provides a non-disturbed annular mixing chamber for generating well-mixed fluid with high angular momentum. The self-cleaning feature on the pintle serves to improve spray quality and increase the life span of the nozzle service, and is especially beneficial to a burner application where cooling of the hot surface of the deflector is needed. In more detail, the self-cleaning feature of the pintle is achieved with a center-drilled hole along the stem of the pintle to the downstream of the deflector plate, where a purge-disk is mounted substantially concentrically to the deflector downstream surface. This forms a passage which can tap part of the atomizing gas from the pressure source and turn it out to become purge gas to the downstream surface of the deflector plate. As the purge gas is exiting out of the slit on the deflector with extremely high velocity, it cleans the surface and prevents recirculated drops from forming the undesired hard-shell-accumulation on the surface that damages the pintle or compromises the nozzle performance.